Process for producing iron-rhodium alloys having improved magnetic transition properties



Dec. 10, 1968 J. 5. KOUVEL 3,415,695

PROCESS FUR PRODUCING IRON-RHODIUM ALLOYS HAVING IMPROVED MAGNETIC TRANSITION PROPERTIES Filed April 29, 1966 In ven tor.-

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i o b United States Patent PROCESS FOR PRODUCING IRON-RHODIUM ALLOYS HAVING IMPROVED MAGNETIC TRANSITION PROPERTIES James S. Kouvel, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 29, 1966, Ser. No. 546,287 4 Claims. (Cl. 148-101) This invention relates to iron-rhodium base alloy bodies exhibiting an antiferromagnetic-ferromagnetic transition and more particularly to an improved process for producing such bodies in which the transition between the antiferromagnetic and ferromagnetic states occurs over a significantly narrower temperature range than previously obtained in bodies produced by existing processes.

The alloys with which the present process is concerned are iron-rhodium base alloys containing CsCl-type ordered crystal structure which may or may not contain additional alloying elements, depending upon the general temperature level at which magnetic transition is desired. These alloys, as earlier indicated, undergo an antiferromagnetic-ferromagnetic transition, which occurs over a 30 to 40 temperature interval. Although transition temperature ranges this broad are acceptable for some applications, narrower transition ranges are usually preferable and are even required in more critical situations such as thermal switches. In addition to iron-rhodium binary alloys, for example, as disclosed in General Electric Research Report No. 61 RL 2870M, November 1961, Kouvel et al., iron-rhodium alloys such as those disclosed in Patent No. 3,144,324 Bither, Jr., Aug. 11, 1964 and Patent No. 3,144,325 Walter, Aug. 11, 1964, constitute examples of alloys with which this invention is concerned. It has now been found that all of the alloys referred to can be processed suitably to exhibit transition temperature ranges significantly narrower than those normally exhibited. This constitutes the principal object of my invention.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawing.

In the drawing:

The figure is a graph showing magnetization (per gram) plotted as a function of temperature for alloy bodies processed according to this invention and in accordance with existing practices.

Generally, the present invention is concerned with an improved process for treating iron-rhodium base alloys to decrease or more sharply define the temperature span over which the body passes from the antiferromagnetic to ferromagnetic state. The process is applicable to ironrhodium base alloys, and this includes iron-rhodium binaries as well as iron-rhodium alloys containing minor amounts of many other metals as is well known in the art. To obtain the improved properties, the alloy is first prepared by melting together the individual constituents and then heat treating the resulting alloy for a long period of time at a temperature of 800 to 1200 C. This heat treatment is carried out to effect homogenization of the alloy. Once the necessary time has elapsed, e.g., 24 to 100 hours, the alloy must then be quenched from a temperature no lower than about 800 C. Under usual conditions the quench would be to room temperature, however, the quench can be terminated at temperatures as high as about 400 C. and still obtain the improved results. Quenching of the alloy is essential to obtainment of the improved magnetic transition properties and is a step having no counterpart in existing methods.

Discussing the process in more detail, alloys were pre- 3,415,695 Patented Dec. 10, 1968 pared from which samples were made to test the effectiveness of the quenching technique for developing an abrupt transition between the antiferromagnetic and ferromagnetic states. The compositions of these alloys are representative of those encompassed within this invention but are intended only by way of illustration and not intended to be limiting. Compositionally, in atom percent, these representative specimens were 48 percent iron-52 percent rhodium, 48 percent iron-50.5 percent rhodium and 1.5 percent palladium, and 48 percent iron-49 percent rhodium and 3 percent platinum. Samples produced from these alloys were then processed by annealing them at 1000 C. for about hours to effect homogenization. At the end of this time, a sample of each of the alloys was removed from the furnace and rapidly quenched to room temperature by immersing it in water. Complete cooling occurred in just a few minutes.

The remaining samples were given a furnace cool, which is the usual practice, and thus required on the order of 24 hours to reach room temperature. All of the specimens were then subjected to a 6 kilogauss field and the magnetization measured while the temperature of the specimens was varied. Referring to the drawing, curves 10, 11 and 12 illustrate the manner in which magnetic transition from the antiferromagnetic to the ferromagnetic state, and vice versa, occurs in the samples which were furnaced cooled. It will be noted that in each case the transition from the antiferromagnetic to ferromagnetic state and back again required a temperature spread of some 30 to 40 C. Curve 11 is that obtained from the iron-rhodium binary while curve 10 was obtained from the alloy containing a small amount of palladium andcurve 12 that obtained from the alloy containing a small amount of platinum. It can be seen that the effect of substituting a small amount of some third or fourth element for part of the rhodium is to shift the overall temperature range at which transition between magnetic states occurs to either a lower or higher temperature, with reference to the binary iron-rhodium alloy. 1

Again referring to the drawing, the curve 10' is that obtained when testing a sample of the same composition as that used for curve 10 but which had been quenched from a temperature above about 800 C. to room temperature. It is obvious from a comparison of the two curves that the transition between the two magnetic states occurs significantly more quickly. Curves 11" and 12 were obtained from materials corresponding exactly with those used to obtain curves 11 and 12 and processed in the same manner except for the quench from some temperature no lower than about 800 C. on down to room temperature. It is apparent that the transition is abrupt rather than comparatively gradual as was the case with those bodies produced by standard practices.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the process of homogenizing by heat treatment, iron-rhodium base alloy bodies containing a CsCl type ordered crystal structure and exhibiting an antiferromagnetic-ferromagnetic transition, comprising the steps of heating said bodies at temperatures above about 800 C. for long periods of time and then slowly cooling said bodies to room temperature, the improvement which consists of rapidly quenching said bodies from a temperature no lower than about 800 C. to about room temperature at the termination of said heating step in order to decrease 'the temperature range over which antiferroma-gneticferromagnetic transition occurs.

2. A process as defined in claim 1 wherein said bodies are quenched in air.

3. A process as defined in claim 1 wherein said bodies are quenched in a liquid.

4. A process as defined in claim 3 wherein said bodies are quenched in water.

References Cited UNITED STATES PATENTS 3,116,182 12/1963 Kouvel 148-103 XR 3,140,941 7/1964 Walter 75134 XR 3,140,942 7/1964 Walter 75172 XR L. DWAYNE RUTLEDGE, Primary Examiner.

PAUL WEINSTEIN, Assistant Examiner.

US. Cl. X.R. 

1. IN THE PROCESS OF HOMOGENIZING BY HEAT TREATMENT, IRON-RHODIUM BASE ALLOY BODIES CONTAINING A CSCL TYPE ORDERED CRYSTAL STRUCTURE AND EXHIBITING AN ANIFERROMAGNETIC-FERROMAGNETIC TRANSITION, COMPRISING THE STEPS OF HEATING SAID BODIES AT TEMPERATURES ABOVE ABOUT 800*C. FOR LONG PERIODS OF TIME AND THEN SLOWLY COOLING SAIID BODIES TO ROOM TEMPERATURE, THE IMPROVEMENT WHICH CONSISTS OF RAPIDLY QUENCHING SAID BODIES FROM A TEMPERATURE NO LOWER THAN ABOUT 800*C. TO ABOUT ROOM TEMPERATURE AT THE TERMINATION OF SAID HEATING STEP IN ORDER TO DECREASE THE TEMPERATURE RANGE OVER WHICH ANTIFERROMAGNETICFERROMAGNETIC TRANSITION OCCURS. 